1. Field of the Invention
The present invention relates to an ink jet recording apparatus and a capping mechanism used therefor. In this respect, the recording is meant to include the provision of ink (printing) or the like for all the ink carriers that may accept it, such as cloths, threads, papers, and sheet materials, and also, it includes not only meaningful images such as characters, but also, meaningless images such as patterning images. Here, the recording apparatus includes all the various information processing apparatus, and printers used therefor as the output equipment thereof. The present invention is applicable to the uses of these apparatuses and equipment.
2. Related Background Art
As the output equipment of a personal computer, copying machine, facsimile apparatus, or the like, there are used recording apparatuses of thermal transfer, LBP, dot impact, ink jet, or other types.
Of these recording apparatuses, the one using ink jet method is given much attention as a printing method superior in quietness in operation. Particularly, the one that utilizes the foaming of liquid by the application of heat provides excellent features in that it easily produces a higher density, operates in a quieter mode, easily meets with coloring requirements, and withstands a higher printing, among others that it is able to demonstrate. Therefore, the use of this apparatus is given much attention as a printing method that provides a higher quality at lower costs.
Also, along with a higher speed required for the operation, it has been generally practiced to use a recording head having a plurality of recording elements being arranged on it (hereinafter, referred to as a multiple head), and also, to use an apparatus provided with a plurality of such multiple heads, which is being developed to meet with increasing demands on recording in colors.
FIG. 6 is a perspective view which shows the principal part of a printer that records on the surface of a sheet by discharging ink from the multiple head substantially downward in the vertical direction. In FIG. 6, a reference numeral 30 designates ink cartridges. These are formed by ink tanks 29 containing ink of four colors, black (Bk), cyan (C), magenta (M), and yellow (Y), respectively, and a multiple head 29. Here, a reference numeral 23 designates a carriage that supports four ink cartridges 30 and enables them to travel and record at the same time. It is arranged that the carriage is on the standby in the home position at h in the location shown in FIG. 6 when recording is at rest or the recovery operation is conducted for the multiple head.
When an instruction is issued to start recording, the carriage 23, which is in the position h (home position) before the recording operation is started, records on the surface of a sheet only by the width D by use of n numbers of multiple nozzles on the multiple head 28 while traveling in the direction x. When data are recorded completely up to the edge portion of the surface of the sheet, the carriage returns to the home position and begins recording again in the direction x. In case of a reciprocal recording, it records while traveling in the direction -x. During the period between the completion of this first recording and the start of the second recording, the sheet is fed in the direction y only by the width D. In this way, the recording for an area only by the width of the multiple head D and the sheet feeding therefor are repeated per carriage scan, thus completing data recording on the surface of one sheet.
Also, in a recording apparatus of the kind, recovery is performed if any non-discharge takes place. Therefore, a suction cap 27, whose use is shared by each of the heads 28, is provided for the recovery operation that is individually executed for each of the heads 28. Also, a pump 27a is provided for exerting negative pressure in this cap 27. With such arrangement, it becomes possible to make the structure simpler and fabricate the system at lower costs than the one having caps and pumps in the same numbers as those of the heads.
Further, in a position between the cap 27 and a recording sheet carrier unit, a wiper blade 31 is provided for wiping and cleaning the leading end of the head 28.
The performance of a recording apparatus that records by discharging ink largely depends on the viscosity of ink to be used. In other words, depending on the viscosity of ink, the discharging amount, discharging speed, the upper limit of driving frequency, and various other characteristics of discharge greatly vary. Then, if the nozzles of an ink jet printer of the kind used for recording are left intact without discharging ink, the surface of nozzles dries to cause ink to be concentrated on the meniscus portion formed on the leading end of nozzles, thus raising the viscosity of ink. On the other hand, there is a limit for the degree of viscosity of ink that allows ink to be discharged in accordance with the energy that can be generated physically. If ink has become overly viscous and exceeds the viscosity limit due to such concentration, discharging is no longer possible. Therefore, non-discharge may ensue in some cases or even if non-discharge may be prevented, dots are twisted to easily invite the degraded quality of recorded images. In order to prevent the degradation of recorded images due to such non-discharge, twisting, or the like, discharges are periodically executed for the nozzles not in use or for the entire nozzles before the elapse of time estimated for encountering any resultant image degradation. In this way, a technique, the so-called "predischarge" that refreshes ink in the nozzle portion for the prevention of ink from becoming extremely over-viscous, is applied to many recording apparatuses. Nevertheless, although this technique is effective when recovering nozzles not in use while in carrying on recording or when recovering nozzles at rest for a short period of time, it should use an enormous amount of ink wastefully to implement the recovery of nozzles that are out of operation for a long period of time or left intact for a long time, because the recovery becomes extremely difficult in such cases. Therefore, this technique can hardly be defined as an appropriate means for the purpose.
Under such circumstances, if no discharge is conducted for a long time, a measure should be taken to prevent ink from being evaporated from the meniscus by covering the nozzle surface by use of some means. A technique of the kind is actually adopted for use by many recording apparatuses (for example, ink jet recording apparatuses manufactured by Canon Inc., Model Nos. BJ10V, BJC-600 and others). If a head protection means of the kind does not perform sufficiently, the over viscosity or drying advances extremely in the nozzles by the time elapses as described above, or additives, dyestuffs, or pigments in ink are educed. As a result, ink is solidified in the vicinity of nozzles, that is, the so-called fixation phenomenon is allowed to occur, there is a need for recovery operations by the application of suction, pressure, or the like as far as such fixation phenomenon is slight. Also, if the fixation phenomenon advances deep into the nozzles, ink supply paths, or other parts in the interior of head, and the fixation becomes extremely strong, no recovery is possible by means of suction, compression purge, or the like, thus causing the head itself to be made unusable in some cases, requiring the replacement thereof. However, since the structure of heads used for ink jet recording apparatuses is complicated, its fabrication is not easy. Therefore, it often costs high, and the replacement of heads leads to the significant increase of running costs. Also, from the viewpoint of reliability enhancement, it is extremely important to prevent ink from being dried in order to avoid its over viscosity and solidified fixation.
In this respect, as a method of preventing ink from becoming overly viscous due to its evaporation, there is disclosed a method of protecting the facing plane of the head by use of a cap in Japanese Patent Laid-Open Application No. 52-138132, for example. Also, as a recent example, a cap is disclosed in Japanese Patent Laid-Open Application No. 5-201009 for the protection of the facing plane.
Also, there has been proposed in recent years a method of exchanging tanks by arranging the structure of the head for discharging ink and the structure of the tank for retaining ink separately for the purpose of curtailing running costs. The apparatus referred to in conjunction with FIG. 6 adopts an ink tank exchanging method of the kind. FIGS. 7A and 7B illustrate the such tank and head in detail. For the recording apparatuses that use the head and tank shown in FIGS. 7A and 7B, a method called "on carriage tank" is adopted to mount a tank 30 on the carriage 23 (see FIG. 6) that holds the head in order to shorten the supply system as much as possible. In case of such tank exchanging method, the interior of the tank 30 is generally divided into a portion where ink 35 is filled as it is, and a portion where an absorbent 33 is filled with ink 35 being absorbed in it as shown in FIGS. 7A and 7B. With such structure, when the ink supply port 28a of the head 28 is in contact with the absorbent 33 under pressure, ink contained in the portion 33a of the absorbent 30, which is being compressed thereby, is squeezed out from the absorbent 30 as the compression is increased. Therefore, in the vicinity of the supply port 28a, there may temporarily exist the ink in a state where no negative pressure is exerted. As a result, the negative pressure, which should be exerted from behind the meniscus in the ordinary use, is absent temporarily in some cases. In the case represented in FIGS. 7A and 7B, the tank is positioned higher than the facing plane 28b of the head, and pressure exerted on the meniscus is positive. Consequently, if the meniscus is broken in this state due to the wetting of the head face, vibration, or the like, ink 35 contained in the head begins to flow out to stop up the conducting hole occasionally.
A numeral 30a denotes an atomosphere communicating hole for communicating the interior of the tank 30 with the atmosphere.
The cap to prevent drying is considered ideal if it can close perfectly when its purpose is only to prevent drying. However, for a cap of a type to airtightly close, the interior of the cap is pressurized when it is used for the head protection, thus allowing the meniscus to be broken. Hence a phenomenon is observed that air is carried into the head or the ink, which is filled in the head, is caused to return to the tank side due to the negative pressure from the tank. Because of this phenomenon, there is a need for supplying ink into the head by means of suction, compression, or the like when the next recording is performed. This invites the increased consumption of wasted ink that is not used for recording per se.
Also, to the contrary, if the cap is released for the execution of recording, the interior becomes negatively pressurized when the cap parts from the facing plane of head. Thus the meniscus is broken and ink in the nozzles is drawn out, leading to a trouble such as wetting of the facing plane of head. Further, if the temperature changes after capping, the air in the cap is caused to expand or contract, resulting in the changes of atmospheric pressure to cause the same trouble as in releasing the cap.
Here, for example, a case where pressure is exerted at the time of capping is given below.
With the dimension of cap (inner dimensions) being: 5 mm laterally, 10 mm longitudinally, and 1 mm high for the rib, the inner volume of the cap will be as follow, provided that the amount of deformation of the cap rib is 0.2 mm after closing the cap: EQU Volume at closing 5.times.10.times.1.0=50 mm.sup.3 EQU Volume after capping 5.times.10.times.0.8=40 mm.sup.3
The atmospheric pressure in the cap changes as shown in the following expression: EQU P.sub.1 V.sub.1 /T.sub.1 =P.sub.2 V.sub.2 /T.sub.2 EQU 1.50/T=P.sub.2.40/T EQU P.sub.2 =50/40=1.25 (atm) (2)
At the time of capping with the structure described above, the interior of the cap is in a state that pressure is exerted at 0.25 atm as compared to the atmospheric pressure.
Also, when pressure is exerted due to the change of temperatures after capping, the atmospheric pressure in the cap changes as the expression given below, provided that the capping is conducted at an outer temperature of 5.degree. C., and then, the temperature rises to 35.degree. C. EQU P.sub.1 V.sub.1 /T.sub.1 =P.sub.2 V.sub.2 /T.sub.2 EQU 1.V/278=P.sub.2.V/308 EQU P.sub.2 =308/278=1.11 (atm) (2)
When the temperature changes as described above, the interior of the cap is in a state that pressure is exerted at 0.11 atm as compared to the atmospheric pressure. Therefore, when closing the cap with such change of temperatures, it is anticipated that the meniscus is broken to cause defective discharges to occur.
In order to avoid the obstacles as described above, a mechanism called an atmospheric conduction hole is arranged for the cap to enable its interior to lead conductively to the air outside, thus avoiding changes of pressure.
For example, an embodiment is disclosed in Japanese Patent Laid-Open Application No. 5-201009, wherein a cap is formed by adhesively bonding two members, while providing an atmospheric conduction hole on the face thus formed by bonding. In other words, as shown in FIGS. 8A and 8B, a cap is structured by a capping rubber 2 and a cap holder 3, and then, a groove is formed on the bottom face of the cap holder 3. In this way, the gap between this groove and a closing member 40 to close the aperture is provided for the atmospheric conduction hole 1. For the purpose of preventing ink from being dried, the diameter of the atmospheric conduction hole 1 should preferably be as small as possible, and its length should preferably as long as possible.
Nevertheless, when ink leakage takes place as described earlier, ink resides in the interior of the cap to clog the atmospheric conduction hole, hence causing ink to drop off in some cases.
Also, there is naturally a limit to receiving and retaining ink by the application of a mode such as proposed in Japanese Patent Laid-Open Application No. 5-201009, in which an atmospheric conduction hole is formed on the face where two members are adhesively bonded, and then, an arrangement is made to utilize the gap between two members formed on both sides of the atmospheric conduction, and to absorb ink by means of capillary force created by use of such gap. Also, since the portion to receive ink is the gap between the two members, the gap should be formed to provide a large capacity inevitably if it is intended to increase the volume of in to be received. In this case, the effectiveness to prevent ink from being dried should be reduced accordingly. On the other hand, it becomes difficult to provide a sufficient volume on the circumference of the atmospheric conduction hole to receive and contain ink if it is intended to sufficiently enhance the effectiveness to prevent ink from being dried. Furthermore, in accordance with this method, leaking ink passes the conduction hole, and conceivably, a small amount of ink resides on the aperture on the head side of the conduction hole, the aperture on the outside air side, and some other parts, respectively. Therefore, this arrangement can hardly be an ideal one as a method of preventing the conduction hole from being clogged.
Also, disclosed in Japanese Patent Laid-Open Application No. 5-201009 as another structure, there is provided a cut off portion formed on its facing plane as a conduction hole, but this cut off portion can hardly be regarded as a desirable mode, because it is difficult to make such hole sufficiently long and thin for the prevention of ink from being dried. There is also a fear that the conduction hole thus formed is clogged by ink adhering to the facing plane. Here, this hole is used together with a mechanism in which a buffer is provided to prevent pressure from changing, and the interior of the cap that covers the facing plane and the buffer is conductively connected by a movable thin film. Under such structure, it is arranged to receive leaking ink by means of the cap. With this arrangement, it is impossible to receive and retain ink in an amount more than the inner volume of the cap. Consequently, if it is intended to secure a sufficient volume for the anticipated ink leakage, there is a need for increasing the inner volume of the cap, while it is required to minimize the volume that may bring about more evaporation from the interior of the cap. Therefore, the cap should be prepared to effectuate two different performances that contradict to each other.
In accordance with such conventional art, the tolerance is extremely low with respect to the anticipated ink leakage. There is a fear, therefore, that any one of these methods cannot cope with the ink leakage that occurs when using the method for exchanging head units of the conventional type where the ink tank and head are integrally formed, and that it is more difficult to cope with the ink leakage that occurs when using the recent tank exchanging method described earlier.